cAMP-dependent protein kinase (cAPK) plays critical roles in different cellular functions. The crystal structure of the catalytic subunit of cAPK is the first protein kinase structure determined and has served as a prototype for the entire protein kinase family, one of the largest protein family. We have also solved the crystal structure of a deletion regulatory subunit using the synchrotron radiation facility at SSRL and this structure has provided valuable insights into how cAPK interact with the second messenger, cAMP. We are now successful in producing crystals of a cAPK holoenzyme complex formed by the catalytic subunit and a deletion regulatory subunit and have collected a complete data set at 3.4 E at home source. Our goal of this proposal is to solve the holoenzyme complex structure at high resolution. Synchrotron radiation at SSRL will allow the collection of the highest possible resolution data, which is necessary for accurate structure determination at high resolution. Solving the holoenzyme complex structure will help us understand the mechanism of cAPK regulation and function.